A multiple stage turbomolecular pump with inter-stage inlet

ABSTRACT

A multiple stage turbomolecular pump is disclosed. The pump comprises: a higher vacuum stage comprising a plurality of rotor blade rows; a lower vacuum stage comprising a plurality of rotor blade rows of a larger diameter than the rotor blade rows of the higher vacuum stage; an inter-stage inlet for inputting gas into the turbomolecular pump between the higher vacuum and lower vacuum stages the inlet extending from an outer circumference of the pump to a radial position inside of the larger diameter and comprising a gas directing formation for directing the gas for entry into the pump in an axial direction.

CROSS-REFERENCE OF RELATED APPLICATION

This application is a Section 371 National Stage Application of international Application No. PCT/GB2018/050074, filed Jan. 11, 2018, and published as WO 2018/134566 A1 on Jul. 26, 2018, the content of which is hereby incorporated by reference in its entirety and which claims priority of British Application No. 1700992.9, filed Jan. 20, 2017.

FIELD

The field of the invention relates to turbomolecular pumps and in particular to multiple stage turbomolecular pumps with an inter-stage inlet.

BACKGROUND

Turbomolecular pumps which provide high and ultrahigh vacuums are known. These pumps are momentum transfer pumps in which gas molecules entering the pump are given momentum by rotating rotor blades. The pump comprises multiple angled rotor and stator blade row pairs, the blades of the rotor blade rows are angled to push the gas molecules towards the exhaust end of the pump. In some cases there may be more rotor blade rows than there are stators such that one or more rotor blade rows may not be paired with a corresponding stator.

Multiple port or split flow pumps have been developed to enable the pumping of several different chambers at different pressures by a single pump. A first stage will receive gas from a high vacuum chamber at a very low pressure, with subsequent stage(s) receiving gas from lower vacuum chambers at higher pressures. Conventionally the gas has entered the pump in a radial direction via an aperture located on the circumference of the pump between two of the stages. Such an aperture requires a reasonable distance between the two stages to provide space for this gas input. FIG. 1 shows such a conventional turbomolecular pump.

It would be desirable to provide compact multiple stage turbomolecular pumps with an inter-stage inlet that provides effective pumping of the different stages.

The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.

SUMMARY

A first aspect of the present invention provides a multiple stage turbomolecular pump comprising: a higher vacuum stage comprising a plurality of rotor blade rows; a lower vacuum stage comprising a plurality of rotor blade rows of a larger diameter than said rotor blade rows of said higher vacuum stage; an inter-stage inlet for inputting gas into said turbomolecular pump between said higher vacuum and lower vacuum stages said inlet extending from an outer circumference of said pump to a radial position inside of said larger diameter and comprising a gas directing formation for directing said gas entering said pump in an axial direction.

The inventors of the present invention recognised that the gap between rotor blade rows of different stages of a multi-stage turbomolecular pump to allow for gas input in a radial direction has led to pumps that are not compact. They also recognised that pumping stages in a turbomolecular pump may have rotor blade rows of increasing diameters in a downstream direction. The rotor diameter of the highest vacuum stage in a turbomolecular pump is governed by the outlet size of the chamber to which it is attached and this may be set by a standard. Subsequent rotor blade rows are not so constrained and may have larger diameters. The difference in size between the rotor blade rows of adjacent stages provides an opportunity for a gas inlet that can direct gas in an axial direction and towards an outer circumference of the larger diameter rotor blade rows. Not only does this reduce the need for a significant gap between the two stages, but it also provides a gas flow directed towards the rotor tips which is the most effective pumping position of the pump where the rotor blades have the highest speed. Thus, a compact pump with improved pumping efficiency is provided.

In some embodiments, said inlet comprises said gas directing formation and at least one aperture, said at least one aperture being located towards a radial outer position of said pump between said larger diameter and a diameter of at least one rotor of said higher vacuum stage.

In some embodiments, said at least one aperture is in a plane substantially perpendicular to an axis of said pump.

Providing the aperture of the inlet in a plane substantially perpendicular to the pump axis at a position that is within the larger diameter allows for gas flowing perpendicularly through this aperture to proceed in an axial direction into the pump and onto the outer tips of this larger diameter rotor. A plane substantially perpendicular to the axis is deemed to be one at an angle of between 70° and 110° of the axis.

In some embodiments, said pump comprises multiple apertures located at different locations around a circumference of said pump.

Although there may only be a single aperture, in some embodiments there are multiple apertures and these are located at different locations around the circumference of the pump thereby improving flow uniformity and increasing the pump's effectiveness and efficiency.

In some embodiments, said at least a portion of said gas directing formation comprises a substantially axially oriented surface.

In some embodiments said gas directing formation comprises a surface that changes from a substantially radially oriented surface at an outer circumference of said pump to a substantially axially oriented surface at a radially innermost point of said gas directing formation.

Although the gas directing formation may have a number of forms, at least a portion of the formation comprises a substantially axially oriented surface, acting to deflect gas moving in a radial direction. A substantially axially oriented surface is considered to be a surface that is at an angle of less than 20° to the axis of the pump. In some embodiments the gas directing formation comprises a surface that changes from a substantially radially oriented surface (one at an angle of less than 20° to a radius of the pump) which brings the gas from a gas source to the pump to a substantially axially oriented surface as the gas enters the pump. In this way the gas flow is delivered towards the pump from a radial direction and the direction of flow is changed to an axial direction before it is actually input into the pump. In other embodiments the gas may be delivered in an axial direction and the gas directing formation does not have a radially oriented surface.

In some embodiments, at least a portion of said gas directing formation is adjacent to or formed by an outer circumference of a portion of said higher vacuum stage.

The different diameters of the different stages of the pump allows at least some of the gas directing formation to lie adjacent to, or be formed by, the outer circumference of the stage with the smaller diameter. In this way the flow of gas can be directed by a formation that is positioned alongside the stage with the smaller diameter and the inlet aperture overlaps with the larger diameter, such that the inlet does not require any substantial gap between the stages allowing for a more compact design of the pump.

In some embodiments, said pump comprises a stator bridge component for bridging between a smaller diameter stator in said higher vacuum stage and a larger diameter stator in said lower vacuum stage, said stator bridge component comprising a ring in a plane substantially perpendicular to the pump axis, said ring comprising multiple apertures.

A stator bridge component is used in multiple stage pumps to bridge between the stators of the different stages which may have different diameters. In embodiments of the invention the stator bridge comprises a ring substantially perpendicular to the pump axis. The ring comprises multiple apertures which are in proximity to the rotor blade of the lower vacuum stage. The multiple apertures of the ring are arranged around the circumference of the ring and form the apertures of the gas inlet.

In some embodiments, an inner diameter of said ring comprises a diameter of a stator of said higher vacuum stage and an outer diameter of said ring comprises a diameter of said stator in said lower vacuum stage.

In order to take advantage of the available space for the input it is advantageous if the ring of the stator bridge extends from a diameter of the stator of the high vacuum stage to a diameter of the larger diameter of the stators in the lower vacuum stage. In addition to the ring in the radial plane (a plane perpendicular to the axis of the pump) the stator bridge has an axial portion which has a height that is set by the distance between the rotor blade rows of the two adjacent stages.

In embodiments the stator bridge has a height that is equal to or greater than the distance between two adjacent rotor blade rows within a stage and equal to or less than the distance between two rotor blade rows that are not adjacent but are separated from each other by a rotor blade row. Owing to the arrangement of the inlet there is no longer a need for a gap between the stages specifically for the input of gas and thus, the stator bridge height which sets the distance between the rotor blade rows of adjacent stages may be such that the rotor blade rows are as close as they are within a stage or at least not more than twice the distance that they are within a stage.

In some embodiments, at least some of said rotor blade rows within said higher vacuum stage have different diameters, a diameter of at least one rotor blade row towards said lower vacuum stage being smaller than a diameter of a rotor blade row that is more remote from said lower vacuum stage.

Although, the rotor blade rows within the different stages may have uniform albeit different diameters to rotor blade rows in other stages, in some embodiments the rotor blade rows within the higher vacuum stage may themselves have different diameters, the diameters of the rotor blade rows getting smaller as one progresses towards the inlet. A smaller diameter towards the inlet provides a larger space for this inlet.

In some embodiments, said rotor blade rows within said higher vacuum stage have diameters that taper towards said lower vacuum stage.

Although the changes in diameter of the rotor blade rows may take a number of forms, in some the change may be successive, the diameters of the rotor blade rows tapering towards the lower vacuum stage.

In some embodiments, each of said rotor blade rows have multiple blades, said blades of said inter-stage inlet rotor blade row, said inter-stage inlet rotor blade row being said rotor blade row of said lower vacuum stage adjacent to said inter-stage inlet, having an angle of an outer portion of said blades adapted for pumping said gas received at said inter-stage inlet and an angle of an inner portion adapted for pumping said gas received at an inlet of said higher vacuum stage.

Owing to the configuration of the pump and in particular to the configuration of the inter-stage inlet the gas being pumped by the higher vacuum stage will enter the lower vacuum stage towards the middle and away from the edge of the rotor blade rows of this stage, while the gas input at the inter-stage inlet will be directed to the outer portion of the rotor blade rows of this stage. Thus, the two gas flows will be, at least in the early portion of this pumping stage, predominantly in different regions of the pump. The inventors recognised that this substantial separation of the flows means that different portions of the rotor blades were predominantly pumping different gas flows. Thus, the blades could be adapted so that the inner portion that predominantly pumped the gas received from the higher vacuum stage are angled in a way that suited this gas flow, while the blades at the outer portion are adapted for pumping the gas received at the inter-stage inlet.

In some embodiments, said blades of said inter-stage inlet rotor have a localised change in geometry between said portion adapted for pumping said gas received at said inter-stage inlet and said portion adapted for pumping said gas received at said higher vacuum stage inlet, said localised change in geometry occurring over a length of said rotor blade that is less than 10% of a radius of said rotor blade.

In some embodiments, said length comprising said localised change in geometry is located within a region of said rotor blade that extends for 20% of said radius of said rotor blade towards a centre of said pump from a radial point inside of said inlet aperture by 5% of said aperture length.

Adapting the blades for the different gas flows means that they will have a localised change of geometry or tilt angle in a region that is between the portion of the blades that predominantly pump one gas flow and the portion that predominantly pumps the other. The localised change in geometry or angle, will occur across a relatively small region. That is it is located within a length that is less than 10% of the rotor blade radius, this length occurring somewhere in a region running inwardly from a radial position for 20% of the radius of the rotor blade and starting at an outer point that starts at a point that is further towards said outer circumference than an inner edge of the inlet aperture by 5% of the length of the radius of the rotor blade.

In some embodiments, said angle of said outer portion of said blades are adapted for increased pumping speed of said gas received at said inter-stage inlet and an angle of an inner portion are adapted for increased compression of said gas received at an inlet of said higher vacuum stage.

The angles of the blades on the rotor blade rows may change across the length of the blade depending on whether the pumping speed or compression of the gas is the most important factor. In some cases, the outer portion of the blades may be adapted for increased pumping speed while the inner portion may be adapted for increased pumping compression. In this regard, the outer circumference will be pumping the gas input at the inter-stage inlet and this may require some acceleration while the inner portion will be pumping gas from an upstream inlet and this may require compression more than it requires acceleration.

In some embodiments, an angle between a radial plane, that is a plane perpendicular to an axis of said pump, and an outer section of said blade is greater than an angle between said radial plane and an inner section of said blade.

As noted previously, the angle of the blade at the outer section is provided for increased pumping speed and thus, generally has more tilt—that is a greater angle between the radial plane and the blade, than the angle of the blade at the inner section which is for providing increased compression. This is different to conventional blades where if there is any change in angle along the length of the blade then generally the outer portion of the blades are flatter to cover the increased area at the outer edge.

In some embodiments, each of said rotor blade rows have multiple blades, said blades of said inter-stage inlet rotor blade row, said inter-stage inlet rotor blade row being said rotor blade row of said lower vacuum stage adjacent to said inter-stage inlet, having at least some of said blades of a smaller diameter than said larger diameter.

In addition to or as an alternative to the blades being differently twisted to allow for the different pumping properties required for the different gases, the length of the rotor blades may also be changed at the rotor blade row adjacent to the inlet. That is some of them may have their rotor tips removed in some cases so that these shortened blades do not extend beyond the inter-stage inlet allowing for a greater inlet area.

In some embodiments, the pump comprises a plurality of stator blade rows, said blades of said inter-stage inlet stator blade row, said inter-stage inlet stator blade row being said stator blade row of said lower vacuum stage adjacent to said inter-stage inlet, having an angle of an outer portion of said blades adapted for the pumping of gas received at said inter-stage inlet and an angle of an inner portion adapted for the pumping of gas received at an inlet of said higher vacuum stage.

In addition to, or as an alternative to, the rotor blades being adapted for differential pumping of the two gases, the stator blades may be adapted in this way. In this regard as the stators do not rotate providing localised changes in geometry may be more straightforward with a stator and thus, in some embodiments may be preferred to changing the rotor geometry.

In some embodiments, said blades of said inter-stage inlet stator have a localised change in geometry between said portion adapted for said gas received at said inter-stage inlet and said portion adapted for said gas received at said upstream inlet, said localised change in geometry occurring over a length of said stator blade that is less than 10% of a radius of said stator blade, said length being in a region of said stator blade that extends for 20% of said radius of said stator blade inwardly from a radial point inside of said inlet aperture by 5% of said aperture length.

In some embodiments, said angle of said outer portion of said blades are adapted for increased pumping speed of said gas received at said inter-stage inlet and an angle of an inner portion are adapted for increased compression of said gas received at an inlet of said higher vacuum stage.

Although the blades may be adapted in a number of ways in some cases it may be desirable for the gas input at the inter-stage inlet to have increased pumping speed while that from the higher vacuum stage may require higher compression.

In some embodiments, each of said stator blade rows have multiple blades, said blades of said inter-stage inlet stator blade row, said inter-stage inlet stator blade row being said stator blade row of said lower vacuum stage adjacent to said inter-stage inlet, having an increased number of blades at an outer diameter compared to a number of blades at an inner diameter of said blade row.

In addition to or as an alternative to the stator blades being twisted, the number of blades towards an outer edge may be different to the number towards the centre, the number being increased towards the outer edge.

As noted above adapting the inter-stage inlet rotor blade row and/or stator blade row may be desirable, and in some embodiments further rotor blade and/or stator blade rows may also be adapted in this way, such that the next stator and/or rotor blade row in the downstream direction may be adapted in a similar or the same way as that of the inter-stage inlet rotor and/or stator blade row.

Although, the inter-stage inlet may be between any of several different stages of the pump, and indeed there may be multiple inter-stage inlets between several different stages, in some embodiments at least one of the inter-stage inlets is between a first high vacuum stage of the pump and a second lower vacuum stage of the pump. The first stage of the pump has a diameter that is dependent on the chamber to which it is attached, and this is often governed by a standard. The rotor blade rows of subsequent stages are not so limited and may have a larger diameter than the first stage.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

The Summary is provided to introduce a selection of concepts in a simplified form that are further described in the Detail Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

FIG. 1 shows a section through a turbomolecular pump with a split flow according to the prior art;

FIG. 2 shows a section ugh a turbomolecular pump with a split flow according to an embodiment;

FIG. 3a shows a cross section through an inlet to a turbomolecular pump according to an embodiment;

FIG. 3b schematically shows a side view of a rotor blade in the inter-stage rotor blade row;

FIG. 4 schematically shows a top view of the rotor blade row;

FIG. 5 shows a stator bridge;

FIG. 6 shows a view of the turbomolecular pump according to an embodiment; and

FIG. 7 shows a plurality of chambers being evacuated by a turbomolecular pump according to an embodiment.

DETAILED DESCRIPTION

Before discussing the embodiments in any more detail, an overview will be provided.

A pump comprising a plurality of pumping stages is disclosed. One pumping stage comprises rotor blade rows of a smaller diameter upstream of a pumping stage with rotor blade rows of a larger rotor diameter. This difference in diameters allows the pump to pump “end on” gas admitted at an inter-stage inlet. The gas is pumped into the stage with the larger diameter rotor blade rows and directed axially onto the exposed outer edge of rotor blades. A special “adapter bridge” may be used between the stator stages for the gas inlet. The pump has the benefit of a splitflow pump, providing the gas with access to the turbo section while reducing pump height as the gas for this interstage is entering on the same axis as the rotor rather than in the more conventional radial method.

Embodiments improve potential pumping performance while using a reduced footprint. This is achieved by using smaller turbo discs in the upper section of the rotor and larger ones below. Such an arrangement is similar to more conventional splitflow rotor arrangements. The difference lies in that the gas enters the inter-stage substantially along the axis of the rotor, in some embodiments through a bespoke “stator bridge” component which exposes the blade tips of the larger diameter turbo stages below. The length of the rotor can be reduced as the blade stages do not have to be spaced apart to enable radial entry of the gas.

FIG. 2 shows the rotor blade rows of a turbomolecular pump according to an embodiment. FIG. 1 shows a similar view of a conventional turbomolecular pump. In FIG. 1 the inter-stage gas inlet 10 between the two stages is a radial inlet with a radial gas flow which therefore requires a significant gap between the rotor blade rows of the two stages to provide a sufficiently large inlet.

In the pump of FIG. 2, the gas input at the inter-stage inlet 10 arrives in a radial direction and is diverted by a gas deflection means (not shown) to an axial direction such that it enters the pump as an axial flow above the upper rotor 22 a of the second stage. The inlet aperture is between the diameter of the rotor blade rows 20 of the first stage and the diameter of the rotor blade rows 22 of the second stage. As can be seen the distance between the first stage with the smaller diameter rotor blade rows and the second stage is considerably smaller in the embodiment of FIG. 2 when compared to the conventional design of FIG. 1. In this regard a typical distance between the stages of embodiments is a distance that is equal to the distance between adjacent rotor blade rows within a stage or is greater than this but no more than the distance between two rotor blade rows within a stage.

The larger distance between the stages of the conventional turbomolecular pump shown in FIG. 1 is required to provide a sufficient sized inlet for gases entering the pump at this stage. The gases in the pump of FIG. 1 enter via a radial input across the top of the upper rotor of the second stage and are diverted from a radial flow to an axial flow by the rotating rotor blade rows of the pump. In the embodiment of FIG. 2 the input gas is diverted by a gas deflecting means at the input and is input in an axial direction onto the outer tips of the rotor blade rows of the second stage. Although the inlet may be smaller than the inlet of FIG. 1, this is compensated for to some extent by the improved pumping effectiveness that results from the input gas being directed onto the outer edge of the rotor blades that rotate faster than the more central portion of these blades.

In this regard, the size of the aperture of the embodiment of FIG. 2 is limited by the difference in diameter size of the rotor blade rows of the different stages. In FIG. 1 the aperture size depends on the distance between the two stages, thus, increasing the aperture size in the pump of FIG. 1 increases the length of the pump.

FIG. 3a shows a cross section through the inter-stage inlet 10 of the turbomolecular pump. Inlet 10 has flow deflecting portion 12 and aperture 14. As can be seen the flow deflection portion takes the form of an axially oriented surface which deflects molecules arriving in a radial direction. The aperture 14 lies in the radial plane perpendicular to the axis of the pump and its size is constrained by the difference in diameters of the two rotor blade rows. The flow deflecting surface 12 runs parallel to the axis adjacent to the outer circumference of the pumping stage comprising the smaller diameter rotor blade rows. In this embodiment aperture 14 is formed within the stator bridge.

FIG. 3b shows a side view of a rotor blade 24 of the inter-stage rotor blade row 22 a. Rotor blade rows of turbomolecular pump have multiple blades that are angled to drive the molecules of the gas being pumped down through the pump. The larger the angle or greater the tilt of the blade the more gas it will capture and the more speed it will impart thereby increasing pumping speed. The flatter the angle the more compression will be provided by the pump.

Owing to the way the gases of the different stages are input to the pump, the gas input to the upstream stage being input across the diameter of the smaller diameter stage, while the gas input at the inter-stage input being input towards an outer edge of the larger diameter stage, the gas from each input have different flows. Thus, following the inter-stage inlet, the two flows of gas at least initially travel as almost separate flows, the newly added gas flowing down the outer edge of the pump and the gas being pumped from an earlier stage being towards a radially inner position of the pump. This allows the blades of either the rotor or stator or both to be designed to be tilted in a way that imparts the required pumping effect to the different gas flows being pumped by the different portions of the rotor blade rows.

FIG. 3b shows a side view of one such rotor blade 24, the wider section indicating the outer portion with in this case a larger tilt angle. The position of inlet aperture 14 is also shown as is the centre axis of the pump C. As can be seen the localised change in geometry occurs over a region R_(L). In embodiments, the length of this region is restricted to 10% or less of the radius R of the blade, and is located somewhere within a region R_(a,) which extends from a point further towards the outer edge of the blade than the inner point of the inlet aperture by 5% of the length of the radius and extends towards a centre of the blade for a length that is 20% or less than the length of the blade. In this way the localised change in geometry occurs in a region where the two gas flows from the different inlets are adjacent/coincide with each other.

The inner section of the blade immediately following the inter-stage inlet will be mainly pumping the gas pumped from the earlier stage while the outer section will be predominantly pumping the gas received at the inter-stage inlet.

In the embodiment of FIG. 3b the design is such that the pumping properties of the blades for the gas input at the inter-stage inlet are focused on capture and acceleration and have a steeper angle of blade. The blades towards the inner portion will be flatter to provide good compression of the gas coming from an earlier stage to avoid any backflow of gases. Thus, the blades will have a twisted nature with a steeper angle towards the outside and a flatter angle towards the inside.

The change in angle of the blades will result in a localised change in geometry around the point where the two flows change predominance. This point will be close to the outer circumference of the rotor of the smaller upstream stage, as moving inside from this point the gas that is predominately being pumped is that from this upstream stage while on the outer edges it is gas being input through the inter-stage inlet. In effect there is a variable geometry with a local change in geometries along the radius of the blades.

Although FIG. 3b shows a change in angle of rotor blade, in some embodiments the localised change in geometry is provided by the stator blades, where again the change in angles is designed to account for the pumping properties required by the predominant gas flow.

Alternatively and/or additionally there may be a change in number of rotor and stator blades towards the outer diameter, the number increasing. In other embodiments the rotor and/or stator diameters may vary towards the inter-stage inlet a tapered diameter being provided, such that they are progressively shorter as the blade rows approach the inter-stage inlet.

FIG. 4 shows a view over the inter-stage rotor blade row 22 a which illustrates the blade's twisted form, and the change in angle that occurs such that in this embodiment the outer portion of blades are more tilted than the inner portion of the blades.

FIG. 5 shows a stator bridge 30 according to an embodiment in cross section. The stator bridge 30 comprises a ring having an inner ring portion 32 with an inner diameter corresponding to the diameter of the smaller blades 20 and an outer ring portion 34 with a diameter configured to surround the larger diameter rotor blades 22. Connecting pieces 33 hold the two rings together, the gaps between these connecting pieces acting as inlet apertures 14.

FIG. 6 shows a view of the turbomolecular pump 5 with the stator bridge 30 of FIG. 5 between the two stages shown.

FIG. 7 shows a plurality of chambers to be evacuated by a turbomolecular pump according to an embodiment. The plurality of chambers 40, 42 and 44 are of increasingly high vacuum. Chamber 40 is evacuated by an initial pump (not shown), while chambers 42 and 44 are evacuated by different stages of turbomolecular pump 5. The highest vacuum stage 44 has an inlet connected to the upper end of the turbomolecular pump 5 that is of a standard size. Chamber 42 has an inlet connected to the inter-stage inlet 10 of pump 5. The inlet 10 comprises an aperture 14 in the transverse plane of the pump and a flow deflecting means 12 in an axial plane perpendicular to a radius of the pump, so that the flow deflecting means is substantially parallel to the pump ax is.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above.

Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims. 

1. A multiple stage turbomolecular pump comprising: a higher vacuum stage comprising a plurality of rotor blade rows; a lower vacuum stage comprising a plurality of rotor blade rows of a larger diameter than said rotor blade rows of said higher vacuum stage; an inter-stage inlet for inputting gas into said turbomolecular pump between said higher vacuum and lower vacuum stages said inlet extending from an outer circumference of said pump to a radial position inside of said larger diameter and comprising a gas directing formation for directing said gas for entry into said pump in an axial direction.
 2. The multiple stage turbomolecular pump according to claim 1, wherein said inlet comprises said gas directing formation and at least one aperture, said at least one aperture being located at a radial outer position of said pump between said larger diameter and a diameter of at least one rotor of said higher vacuum stage.
 3. The multiple stage turbomolecular pump according to claim 2, wherein said at least one aperture is in a plane substantially perpendicular to an axis of said pump.
 4. The multiple stage turbomolecular pump according to claim 2, wherein said pump comprises multiple apertures located at different locations around a circumference of said pump.
 5. The multiple stage turbomolecular pump according to claim 1, wherein at least a portion of said gas directing formation comprises a substantially axially oriented surface wherein said gas directing formation comprises a surface that changes from a substantially radially oriented surface at an outer circumference of said pump to a substantially axially oriented surface at a radially innermost point of said gas directing formation.
 6. (canceled)
 7. The multiple stage turbomolecular pump according to claim 1, wherein at least a portion of said gas directing formation is adjacent to or formed by an outer circumference of a portion of said higher vacuum stage.
 8. The multiple stage turbomolecular pump according to claim 1, said pump comprises a stator bridge component for bridging between a smaller diameter stator in said higher vacuum stage and a larger diameter stator in said lower vacuum stage, said stator bridge component comprising a ring in a plane substantially perpendicular to an axis of said pump, said ring comprising multiple apertures.
 9. The multiple stage turbomolecular pump according to claim 8, wherein an inner diameter of said ring comprises a diameter of a stator of said higher vacuum stage and an outer diameter of said ring comprises a larger diameter of said stator in said lower vacuum stage.
 10. The multiple stage turbomolecular pump according to claim 7, wherein said stator bridge comprises a height that is equal to or greater than the distance between two adjacent rotor blade rows and equal to or less than the distance between two rotor blade rows that are not adjacent but are separated from each other by a rotor blade row.
 11. The multiple stage turbomolecular pump according to claim 1, wherein at least some of said rotor blade rows within said higher vacuum stage have different diameters, a diameter of at least one rotor towards said lower vacuum stage being smaller than a diameter of a rotor that is more remote from said lower vacuum stage, wherein said rotor blade rows within said higher vacuum stage have diameters that taper towards said lower vacuum stage.
 12. (canceled)
 13. The multiple stage turbomolecular pump according to claim 1, wherein each of said rotor blade rows have multiple blades, said blades of said inter-stage inlet rotor blade row, said inter-stage inlet rotor blade row being said rotor blade row of said lower vacuum stage adjacent to said inter-stage inlet, having an angle of an outer portion of said blades adapted for pumping said gas received at said inter-stage inlet and an angle of an inner portion adapted for pumping said gas received at an inlet of said higher vacuum stage.
 14. The multiple stage turbomolecular pump according to claim 13, wherein said blades of said inter-stage inlet rotor have a localised change in geometry between said portion adapted for pumping said gas received at said inter-stage inlet and said portion adapted for pumping said gas received at said higher vacuum stage inlet, said localised change in geometry occurring over a length of said rotor blade that is less than 10% of a radius of said rotor blade, said length of said rotor being in a region of said rotor blade that extends for 20% of said radius of said rotor blade inwardly from a radial point inside of said inlet aperture by 5% of said aperture length.
 15. (canceled)
 16. The multiple stage turbomolecular pump according to claim 13, wherein said angle of said outer portion of said blades are adapted for increased pumping speed of said gas received at said inter-stage inlet and said angle of said inner portion are adapted for increased compression of said gas received at an inlet of said higher vacuum stage, wherein an angle between a radial plane, that is a plane perpendicular to said axis of said pump, and said outer portion of said blade is greater than an angle between said radial plane and said inner portion of said blade.
 17. (canceled)
 18. The multiple stage turbomolecular pump according to claim 1, wherein each of said rotor blade rows have multiple blades, said blades of said inter-stage inlet rotor blade row, said inter-stage inlet rotor blade row being said rotor blade row of said lower vacuum stage adjacent to said inter-stage inlet, having at least some of said blades of a smaller diameter than said larger diameter.
 19. The multiple stage turbomolecular pump according to claim 1, comprising a plurality of stator blade rows, said blades of said inter-stage inlet stator blade row, said inter-stage inlet stator blade row being said stator blade row of said lower vacuum stage adjacent to said inter-stage inlet, having an angle of an outer portion of said blades adapted for the pumping of gas received at said inter-stage inlet and an angle of an inner portion adapted for the pumping of gas received at an inlet of said higher vacuum stage.
 20. The multiple stage turbomolecular pump according to claim 19, wherein said blades of said inter-stage inlet stator have a localised change in geometry between said portion adapted for said gas received at said inter-stage inlet and said portion adapted for said gas received at said higher vacuum stage inlet, said localised change in geometry occurring over a length of said stator blade that is less than 10% of a radius of said stator blade.
 21. The multiple stage turbomolecular pump according to claim 20, said length being in a region of said stator blade that extends for 20% of said radius of said stator blade inwardly from a radial point inside of said inlet aperture by 5% of said aperture length.
 22. The multiple stage turbomolecular pump according to claim 19, wherein said angle of said outer portion of said blades are adapted for increased pumping speed of said gas received at said inter-stage inlet and an angle of an inner portion are adapted for increased compression of said gas received at an inlet of said higher vacuum stage.
 23. The multiple stage turbomolecular pump according to claim 1, wherein each of said stator blade rows have multiple blades, said blades of said inter-stage inlet stator blade row, said inter-stage inlet stator blade row being said stator blade row of said lower vacuum stage adjacent to said inter-stage inlet, having an increased number of blades at an outer diameter compared to a number of blades at an inner diameter of said blade row.
 24. The multiple stage turbomolecular pump according to claim 1, wherein said higher vacuum stage is a first stage of said pump and said lower vacuum stage is a second stage of said pump, said first stage comprising an inlet that extends across a diameter of said rotor blade rows of said first stage. 